Cobalt‐Based Metal–Organic Framework Nanoarrays as Bifunctional Oxygen Electrocatalysts for Rechargeable Zn‐Air Batteries

Abstract

Owing to their high theoretical energy density, environmental benign character, and low cost, rechargeable Zn-air batteries have emerged as an attractive energy technology. Unfortunately, their energy efficiency is seriously plagued by sluggish oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) that alternately occurs on air electrodes. Herein, we demonstrate Co-based metal-organic framework (Co(bpdc)(H O) (bpdc=biphenyl -4, 4'-dicarboxylic acid), Co-MOF) arrays as novel bifunctional oxygen electrocatalysts. The Co-MOF is in situ constructed on a three-dimensional graphite foam (GF) through a hydrothermal reaction. In a 1 m KOH aqueous solution, the resultant Co-MOF/GF exhibits an OER overpotential of only ≈220 mV at 10 mA cm , which is much lower than those for Ir/C and previously reported noble metal-free electrocatalysts. In conjunction with its ORR half-wave potential of 0.7 V (vs. RHE), the Co-MOF/GF manifests a greatly decreased potential gap of ≈0.75 V in comparison with Pt/C-Ir/C couple and previously reported bifunctional oxygen electrocatalysts. Furthermore, an assembled rechargeable zinc-air battery using Co-MOF electrocatalyst in an air electrode delivers a maximum power density of 86.2 mW cm and superior charge-discharge performance. Microscopic, spectroscopic and electrochemical analyses prove that the initial Co-MOF is transformed into Co-oxyhydroxides during the OER and ORR process, which essentially serve as bifunctional active centers.

A Porphyrinic Zirconium Metal–Organic Framework for Oxygen Reduction Reaction: Tailoring the Spacing between Active-Sites through Chain-Based Inorganic Building Units

“…We further compared the peak power density, which is one of the critical metrics for practical applications, to those of other electrodes. The peak power density of PCN-226(Co) was 133 mW cm –2 , which was comparable to the 150 mW cm –2 observed for Pt/C+RuO 2 ( Figure 4 d) and exceeded those for the previously reported state-of-the-art noble metal-free electrocatalysts, such as MOFs and MOF derived composites, 60 − 64 carbon materials, 11 , 65 − 69 and metal oxide composites 70 − 72 ( Table S8 ). Significantly, no noticeable performance decrease was observed after 550 charge–discharge cycles over 160 h, which indicated a high electrocatalytic durability ( Figure 4 e).…”

Synthesis and Optimization of Zeolitic Imidazolate Frameworks for the Oxygen Evolution Reaction

“…Similarly, the XPS spectra of Co 2p in the parent ZIF‐67 sample showed two peaks at around 782 eV and 797.7 eV, which shift to lower binding energy without satellite peaks in the sample used for the OER test for 24 h, supporting the formation of the cobalt oxide/hydroxide phase (Figure S18 in the Supporting Information) [44–46] . XRD profiles and SEM images further indicate the transformation of crystalline ZIF‐67 to an amorphous phase (Figure S19 in the Supporting Information), in good agreement with earlier reports of MOF/ZIF‐based OER catalysts (Figure S20 in the Supporting Information) [44, 47] . Overall, such results suggest that ZIF‐67 is a promising and inexpensive catalyst, and its activity for OER or ORR or both can be further improved by tailoring the framework/micro/nano structure by the strategic design of MOFs/ZIFs [9, 10, 13, 26] …”

Rational Design of Nanoarray Architectures for Electrocatalytic Water Splitting

“…To maximize the catalytic performance, the highly reactive oxygen species generated by the low‐temperature air plasma activate the metal sites in the Fe/Co bimetallic cyanide framework for OER by oxygen bonding to the metal sites in the nanoporous framework, bringing new opportunity to develop framework‐based electrocatalysts . Co‐based metal–organic framework (Co(bpdc)(H 2 O) 4 ; bpdc = biphenyl‐4,4′‐dicarboxylic acid) arrays for OER reached current density of 10 mA cm −2 at a small overpotential of 220 mV and a low Tafel slope (72 mV per decade) in alkaline media . With regard to MOFs as promising electrocatalysts, MOF [{Fe 3 (µ 3 ‐O)(bdc) 3 } 4 {Co 2 (na) 4 (H 2 O) 2/3 (OH) 4/3 } 3 ] on Ni foam presented current density of 10 mA cm −2 at a small overpotential of 225 mV and a low Tafel slope (48 mV per decade) for excellent and stable OER in alkaline electrolyte .…”

“…Cobalt phosphosulfide (Co 2− x SP) nanosheet on carbon fiber paper substrate by a simple two‐step electrodeposition route showed current density of 10 mA cm −2 at a low overpotential (279 mV), a low Tafel slope (54 mV per decade), and robust stability toward remarkable OER activity . Beyond metal‐based arrays, carbon‐based electrocatalysts and the integrated arrays using carbon‐based materials and metal‐based arrays have also presented considerably enhanced electrocatalytic performance . Although various inorganic material arrays have been extensively investigated, the integration of inorganic arrays and carbon‐based layers displayed higher electrocatalytic performance than single‐phase inorganic material array or carbon‐based layers.…”